Semiconductor film, solar cell, light-emitting diode, thin film transistor, and electronic device

ABSTRACT

A semiconductor film includes a cluster of semiconductor quantum dots each having a metal atom and ligands coordinating to respective semiconductor quantum dots, and the semiconductor quantum dots have an average shortest inter-dot distance of less than 0.45 nm. A solar cell, a light-emitting diode, a thin film transistor, and an electronic device include the semiconductor film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of InternationalApplication No. PCT/JP2013/080407, filed Nov. 11, 2013, the disclosureof which is incorporated herein by reference in its entirety. Further,this application claims priority from Japanese Patent Application No.2012-283030, filed Dec. 26, 2012, the disclosure of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a semiconductor film, a solar cell, alight-emitting diode, a thin film transistor, and an electronic device.

BACKGROUND ART

In recent years, high-efficiency solar cells referred to asthird-generation solar cells have been actively studied. Among them,solar cells in which colloidal quantum dots are used are reported to beable to increase the quantum efficiency due to, for example, themultiexciton generation effect, and they are attracting attention.However, solar cells in which colloidal quantum dots are used (alsoreferred to as quantum dot solar cells) exhibit a conversion efficiencyof about 7% at most, and a further increase in the conversion efficiencyis desired.

In such quantum dot solar cells, a semiconductor film constituted of acluster of quantum dots serves as a photoelectric conversion layer, and,therefore, semiconductor films constituted of a cluster of quantum dotsare themselves being actively studied.

For example, semiconductor nanoparticles in which relatively longligands having six or more hydrocarbon groups are used are disclosed(for example, see Japanese Patent No. 4425470).

With respect to methods for improving the characteristics ofsemiconductor films constituted of a cluster of quantum dots, it isreported that replacing ligand molecules bonded to quantum dots (forexample, about 2 nm to about 10 nm) by shorter ligand moleculesincreases electrical conductivity (for example, see S. Geyer, et al.,“Charge transport in mixed CdSe and CdTe colloidal nanocrystal films”,Physical Review B (2010)). In “Structural, Optical, and ElectricalProperties of Self-Assembled Films of PbSe Nanocrystals Treated with1,2-Ethanedithiol” (J. M. Luther, et al., ACS Nano (2008)), it isreported that replacing oleic acid (having a molecular chain length ofabout 2 nm to about 3 nm) around PbSe quantum dots by ethanedithiol(having a molecular chain length of 1 nm or shorter) causes quantum dotsto come closer to each other, thereby increasing the electricalconductivity.

SUMMARY OF INVENTION

However, the semiconductor film described in Japanese Patent No. 4425470includes large ligands, and the degree to which semiconductor quantumdots come close to each other is insufficient. Therefore, thephotoelectric conversion characteristics of the semiconductor filmdescribed in Japanese Patent No. 4425470 are not favorable. Also whenbutylamine, which is used in “Charge transport in mixed CdSe and CdTecolloidal nanocrystal films” (S. Geyer, et al., Physical Review B(2010)), or ethanedithiol, which is used in “Structural, Optical, andElectrical Properties of Self-Assembled Films of PbSe NanocrystalsTreated with 1,2-Ethanedithiol” (J. M. Luther, et al., ACS Nano (2008)),is used as ligands, a photocurrent value of only about several hundredsof nanoamperes at most can be obtained, according to, for example,“Charge transport in mixed CdSe and CdTe colloidal nanocrystal films”(S. Geyer, et al., Physical Review B (2010)). Further, whenethanedithiol is used as ligands, film detachment of the semiconductorfilm easily occurs.

The present invention addresses provision of a semiconductor film withwhich a high photocurrent value can be achieved and in which occurrenceof film detachment is suppressed.

The invention also addresses provision of a solar cell, a light-emittingdiode, a thin film transistor, and an electronic device with which ahigh photocurrent value can be achieved and in which occurrence of filmdetachment is suppressed.

Aspects of the invention include the following:

<1> A semiconductor film which includes a cluster of semiconductorquantum dots each having a metal atom and ligands coordinating torespective semiconductor quantum dots, the semiconductor quantum dotshaving an average shortest inter-dot distance of less than 0.45 nm.

<2> The semiconductor film according to <1>, in which the semiconductorquantum dots have an average shortest inter-dot distance of less than0.30 nm.

<3> The semiconductor film according to <1>, in which the semiconductorquantum dots have an average shortest inter-dot distance of less than0.20 nm.

<4> The semiconductor film according to any one of <1> to <3>, in whichthe semiconductor quantum dots are at least one selected from the groupconsisting of PbS, PbSe, InN, InAs, InSb, and InP.

<5> The semiconductor film according to any one of <1> to <4>, in whichan average particle diameter of the semiconductor quantum dots is from 2nm to 15 nm.

<6> The semiconductor film according to <4> or <5>, in which thesemiconductor quantum dots are PbS.

<7> A solar cell including the semiconductor film according to any oneof <1> to <6>.

<8> A light-emitting diode including the semiconductor film according toany one of <122 to <6>.

<9> A thin film transistor including the semiconductor film according toany one of <122 to <6>.

<10> An electronic device including the semiconductor film according toany one of <122 to <6>.

According to the invention, a semiconductor film with which a highphotocurrent value can be achieved and in which film detachment issuppressed can be provided.

According to the invention, a solar cell, a light-emitting diode, a thinfilm transistor, and an electronic device with which a high photocurrentvalue can be achieved and in which film detachment is suppressed can beprovided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating an example of a configuration ofa p-n junction solar cell in which a semiconductor film according to theinvention is employed.

FIG. 2 is a view illustrating an in-plane angle dependency of scatteredlight in grazing incidence small angle X-ray scattering measurements ofthe semiconductor films manufactured in Examples and ComparativeExamples.

FIG. 3 is a schematic view illustrating a comb-shaped electrodesubstrate used in Examples.

FIG. 4 is a schematic view illustrating a method of irradiating thesemiconductor film manufactured in Examples with a monochromic light.

FIG. 5 is a view illustrating a relationship between a distance betweensemiconductor quantum dot particles and an electrical conductancemeasured in Examples.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a semiconductor film and a method of manufacturing thesemiconductor film according to the invention are described in detail.

<Semiconductor Film>

A semiconductor film according to the invention includes a cluster ofsemiconductor quantum dots each having a metal atom and ligandscoordinating to respective semiconductor quantum dots, and thesemiconductor quantum dots have an average shortest inter-dot distanceof less than 0.45 nm.

The semiconductor quantum dots are semiconductor particles that areconfigured to contain a metal atom, and that are nano-sized particleshaving a particle diameter of several nanometers to several tens ofnanometers. The cluster of semiconductor quantum dots refers to astructure in which many semiconductor quantum dots (for example, 100 ormore semiconductor quantum dots per 1 μm² square) are arranged in closeproximity to each other.

The “semiconductor” as used in the invention means a substance having aspecific resistance value of from 10⁻² Ωcm to 10⁸ Ωcm.

In the invention, the scope of the metal atoms for constitutingsemiconductor quantum dots include semimetal atoms, a representativeexample of which is a Si atom.

Examples of semiconductor quantum dot materials for constitutingsemiconductor quantum dots include nanoparticles (particles having asize of from 0.5 nm to less than 100 nm) of a general semiconductorcrystal [a) a group IV semiconductor, b) a compound semiconductor ofgroup IV-IV, group III-V, or group II-VI, and c) a compoundsemiconductor composed of a combination of three or more selected fromgroup II, group III, group IV, group V, and group VI elements]. Specificexamples of semiconductor quantum dot materials include semiconductormaterials having a relatively narrow band gap, such as PbS, PbSe, InN,InAs, Ge, InAs, InGaAs, CuInS, CuInSe, CuInGaSe, InSb, Si, and InP.

The semiconductor quantum dots include at least one semiconductorquantum dot material.

The semiconductor quantum dot material preferably has a bulk band gap of1.5 eV or smaller. Using such a semiconductor material having arelatively narrow band gap enables achievement of a high conversionefficiency when, for example, the semiconductor film according to theinvention is used as a photoelectric conversion layer of a solar cell.

The semiconductor quantum dots may have a core-shell structure in whicha semiconductor quantum dot material serves as a core, and in which thesemiconductor quantum dot material is covered with a coating compound.Examples of the coating compound include ZnS, ZnSe, ZnTe, and ZnCdS.

Among the above materials, the semiconductor quantum dot material ispreferably PbS or PbSe due to ease of synthesis of semiconductor quantumdots thereof. Using InN is also preferable in consideration of smallenvironmental load thereof.

When the semiconductor film according to the invention is used in solarcell applications, it is preferable that the semiconductor quantum dotshave a still narrower band gap with a view to enhancement of thephotoelectric conversion efficiency due to the multiexciton generationeffect. More specifically, the band gap is preferably 1.0 eV or smaller.

From the viewpoint of obtaining a still narrower band gap and enhancingthe multiexciton generation effect, the semiconductor quantum dotmaterial is preferably PbS, PbSe, or InSb.

The average particle diameter of the semiconductor quantum dots ispreferably from 2 nm to 15 nm. The average particle diameter of thesemiconductor quantum dots means the average particle diameter of tensemiconductor quantum dots. The particle diameters of semiconductorquantum dots can be measured using a transmission electron microscope.The “average particle diameter” of semiconductor quantum dots in thepresent specification refers to a number average particle diameter,unless specified otherwise. Namely, the number average particle diameterof the semiconductor quantum dots is preferably from 2 nm to 15 nm.

Semiconductor quantum dots generally include particles having varioussizes of from several nanometers to several tens of nanometers. Insemiconductor quantum dots, when the average particle diameter of thequantum dots is decreased to a size that is equal to or smaller than theBohr radius of electrons that are inherently present therein, aphenomenon that the band gap of the semiconductor quantum dots changesdue to the quantum size effect occurs. For example, the Bohr radius isrelatively large in group II-VI semiconductors, and the Bohr radius inPbS is said to be about 18 nm. InP, which is a III-V groupsemiconductor, is said to have a Bohr radius of about 10 nm to about 14nm.

Accordingly, when the average particle diameter of the semiconductorquantum dots is, for example, 15 nm or less, the band gap can becontrolled by the quantum size effect.

In particular, when the semiconductor film according to the invention isapplied to a solar cell, it is important that the band gap be adjustedto the optimum value through the quantum size effect, regardless of whatsemiconductor quantum dot material is used. In general, the smaller theaverage particle diameter of the semiconductor quantum dots is, thelarger the band gap is. Therefore, when the average particle diameter ofthe semiconductor quantum dots is 10 nm or less, a larger change in theband gap can be expected. Therefore, the size (number average particlediameter) of the quantum dots is preferably 10 nm or less since the bandgap thereof can easily be adjusted to a value optimum for the spectrumof sunlight even when the semiconductor quantum dots are a narrow-gapsemiconductor. When the average particle diameter of the semiconductorquantum dots is small and quantum confinement is remarkable, there isalso an advantage in that an increase in the multiexciton generationeffect can be expected.

The average particle diameter (number average particle diameter) of thesemiconductor quantum dots is preferably 2 nm or more. When the averageparticle diameter of the semiconductor quantum dots is 2 nm or more, thequantum confinement effect is not excessively strong, and the band gapcan easily be adjusted to the optimum value. Setting the averageparticle diameter of the semiconductor quantum dots to be 2 nm or moremakes it easy to control the crystal growth of the semiconductor quantumdots during the synthesis of the semiconductor quantum dots.

In the film configured by including a cluster of semiconductor quantumdots, a large inter-dot distance (spacing) between the semiconductorquantum dots leads to a decrease in the electrical conductivity, and thefilm becomes an insulator. Decreasing the inter-dot distance between thesemiconductor quantum dots improves the electrical conductivity, andenables a semiconductor film having a high photocurrent value to beobtained.

Adopting a configuration in which a semiconductor film includes acluster of semiconductor quantum dots each having a metal atom andspecific ligands coordinating to respective semiconductor quantum dotsenables an average shortest distance between the dots of less than 0.45nm.

Here, the average shortest inter-dot distance between semiconductorquantum dots means the average value of the shortest distance betweenthe surface of one semiconductor quantum dot A and the surface ofanother semiconductor quantum dot B adjacent to the semiconductorquantum dot A. More specifically, the average shortest inter-dotdistance is calculated as follows.

The average shortest inter-dot distance between semiconductor quantumdots can be obtained by structure evaluation of a quantum dot filmincluding semiconductor quantum dots using grazing incidence small angleX-ray scattering (GISAXS). Through this measurement, a center-to-centerdistance d between adjacent semiconductor quantum dots can be obtained.The shortest inter-dot distance is calculated by subtracting theparticle diameter of the semiconductor quantum dot from the obtainedcenter-to-center distance d.

In the structure evaluation of the semiconductor film using a GISAXSmeasuring instrument, the average of scattered X-rays from thesemiconductor quantum dots present in the entire region irradiated withX-rays is detected as scattered X-rays from the object to be measured.The shortest inter-dot distance calculated from the detected scatteredX-rays is the “average shortest inter-dot distance”, which is theaverage value of the shortest inter-dot distances.

It is conceivable that that the smaller the average shortest inter-dotdistance of semiconductor quantum dots is, the greater the photocurrentvalue of the semiconductor film is. However, a configuration in whichthe average shortest inter-dot distance is 0 nm, i.e., a configurationin which semiconductor quantum dots contact with one another andaggregate, is equivalent to a bulk semiconductor, and thecharacteristics of nanosized semiconductor quantum dots cannot beobtained with such a configuration. Therefore, the average shortestinter-dot distance of semiconductor quantum dots is preferably more than0 nm.

In the invention, the average shortest inter-dot distance of thesemiconductor quantum dot is more preferably less than 0.30 nm, andstill more preferably less than 0.20 nm.

Although the thickness of the semiconductor film is not particularlylimited, the thickness of the semiconductor film is preferably 10 nm ormore, and more preferably 50 nm or more, from the viewpoint of obtaininghigh electrical conductivity. However, the thickness of thesemiconductor film is preferably 300 nm or less in consideration ofpreventing the carrier concentration from becoming too high andfacilitating manufacturing.

<Method of Manufacturing Semiconductor Film>

Methods employed for manufacturing a semiconductor film according to theinvention are not particularly limited. From the viewpoint of furtherdecreasing the spacing between the semiconductor quantum dots anddensely arranging the semiconductor quantum dots, a method is preferablewhich includes forming a cluster of semiconductor quantum dots using asemiconductor quantum dot dispersion liquid that includes semiconductorquantum dots to which first ligands having a relatively long molecularchain coordinate, and thereafter replacing the first ligands by specificligands (second ligands) having a shorter molecular chain than that ofthe first ligands, thereby decreasing the spacing between thesemiconductor quantum dots.

Methods of manufacturing a semiconductor film according to the inventionare specifically described below.

(1) First Embodiment

A method of manufacturing a semiconductor film according to a firstembodiment includes:

a semiconductor quantum dot cluster formation process of applying asemiconductor quantum dot dispersion liquid onto a substrate to form acluster of semiconductor quantum dots, the semiconductor quantum dotdispersion liquid including semiconductor quantum dots each having ametal atom, first ligands coordinating to respective semiconductorquantum dots, and a first solvent; and

a ligand exchange process of applying, to the cluster of semiconductorquantum dots, a solution that includes second ligands (hereinafter alsoreferred to as “specific amine-based ligands”) and a second solvent,thereby replacing the first ligands, which coordinate to thesemiconductor quantum dots, by the second ligands, the second ligandshaving a molecular chain length shorter than that of the first ligands,and the second ligands being of at least one kind selected from thegroup consisting of a ligand represented by Formula (A), a ligandrepresented by Formula (B), and a ligand represented by Formula (C).

In Formula (A), X¹ represents —SH, —NH₂, or —OH, and each of A¹ and B¹independently represents a hydrogen atom or a substituent having from 1to 10 atoms, provided that, when both A¹ and B¹ are hydrogen atoms, X¹represents —SH or —OH.

In Formula (B), X² represents −SH, −NH₂, or −OH, and each of A² and B²independently represents a hydrogen atom or a substituent having from 1to 10 atoms.

In Formula (C), A³ represents a hydrogen atom or a substituent havingfrom 1 to 10 atoms.

In the semiconductor quantum dot cluster formation process in the methodof manufacturing a semiconductor film according to the presentembodiment, the semiconductor quantum dot dispersion liquid is appliedonto a substrate, thereby forming a cluster of semiconductor quantumdots on the substrate. In this process, since the semiconductor quantumdots are dispersed in the first solvent due to the presence of the firstligands having a longer molecular chain length than that of the secondligands, the semiconductor quantum dots have lower tendency to take anaggregated bulk form. Accordingly, the cluster of semiconductor quantumdots can take a configuration in which the semiconductor quantum dotsare individually arranged, when the semiconductor quantum dot dispersionliquid is applied to a substrate.

Subsequently, in the ligand exchange process, the solution of thespecific amine-based ligands is applied to the cluster of semiconductorquantum dots, whereby the first ligands having a longer molecular chainlength than that of the second ligands and coordinating to thesemiconductor quantum dots are replaced by the second ligands (specificamine-based ligands). The specific amine-based ligands have an aminogroup in a molecule thereof, as illustrated in Formulae (A) to (C). Anamino group has a high complex stability constant, and, in Formulae (A)and (B), it is conceivable that amino groups promotes formation of acomplex of a metal atom in each semiconductor quantum dot and —SH, —NH₂,or —OH represented by X¹ (or X²). In Formula (C), it is conceivable thatthe amino group promotes formation of a complex of a metal atom in eachsemiconductor quantum dot and OH in the carboxy group. Accordingly, itis conceivable that the second ligands (specific amine-based ligands)coordinate in place of the first ligands having a longer molecular chainlength than that of the second ligands, and that the second ligands formcoordination bonds with semiconductor quantum dots, thereby making iteasier for the semiconductor quantum dots to come closer to each other.It is conceivable that the semiconductor quantum dots coming closer toeach other increases the electrical conductivity of the cluster ofsemiconductor quantum dots, and enables provision of a semiconductorfilm having a high photocurrent value.

It is conceivable that chelate coordination of the specific ligandsreduces an influence from steric hindrance, and makes the semiconductorquantum dots to come quite close to each other, as a result of which thecluster of semiconductor quantum dots becomes a strong semiconductorfilm, and becomes difficult to detach from the substrate.

The respective processes are specifically described below.

Semiconductor Quantum Dot Cluster Formation Process

In the semiconductor quantum dot cluster formation process, thesemiconductor quantum dot dispersion liquid that includes semiconductorquantum dots, the first ligands coordinating to respective semiconductorquantum dots, and the first solvent is applied to a substrate, to form acluster of semiconductor quantum dots.

The semiconductor quantum dot dispersion liquid may be applied to eithera surface of the substrate or another layer provided on the substrate.

Examples of another layer provided on the substrate include an adhesionlayer for improving the adhesion between the substrate and the clusterof semiconductor quantum dots, and a transparent conductive layer.

—Semiconductor Quantum Dot Dispersion Liquid—

The semiconductor quantum dot dispersion liquid includes semiconductorquantum dots each having a metal atom, the first ligands, and the firstsolvent.

The semiconductor quantum dot dispersion liquid may further includeother components as far as the effects of the invention are notimpaired.

(Semiconductor Quantum Dot)

The details of the semiconductor quantum dots containing a metal atom inthe semiconductor quantum dot dispersion liquid are as described above,and preferred embodiments thereof are also the same as those describedabove.

The content of semiconductor quantum dots in the semiconductor quantumdot dispersion liquid is preferably from 1 mg/ml to 100 mg/ml, and morepreferably from 5 mg/ml to 40 mg/ml.

A content of semiconductor quantum dots in the semiconductor quantum dotdispersion liquid of 1 mg/ml or higher provides a high semiconductorquantum dot density on the substrate, and makes it easy to obtain afavorable film. Meanwhile, a content of semiconductor quantum dots of100 mg/ml or lower makes the thickness of a film obtained by one timeapplication of the semiconductor quantum dot dispersion liquid lesslikely to become large. Therefore, ligand exchange from the firstligands coordinating to respective semiconductor quantum dots in thefilm can sufficiently be performed.

(First Ligands)

The first ligands contained in the semiconductor quantum dot dispersionliquid work as ligands coordinating to respective semiconductor quantumdots, and also serve as a dispersant for dispersing the semiconductorquantum dots in the first solvent due to their molecular structure,which tends to cause steric hindrance.

The molecular chain length of the first ligands is longer than that ofthe after-mentioned second ligands. When there is a branched structurein the molecule, whether the molecular chain length is longer or shorteris determined from the length of the main chain. The ligands representedby Formula (A), the ligands represented by Formula (B), and the ligandsrepresented by Formula (C), which can work as the second ligands, areinherently hard to disperse in organic solvent systems, and theseligands do not correspond to the first ligands. Here, the dispersionrefers to a state in which sedimentation of particles or clouding doesnot occur.

From the viewpoint of improving the dispersion of the semiconductorquantum dots, the first ligands are preferably ligands each having amain chain having 6 or more carbon atoms, and more preferably ligandseach having a main chain having 10 or more carbon atoms.

Specifically, the first ligands may be a saturated compound or anunsaturated compound, and examples thereof include decanoic acid, lauricacid, myristic acid, palmitic acid, stearic acid, behenic acid, oleicacid, erucic acid, oleylamine, dodecylamine, dodecanethiol,1,2-hexadecanethiol, trioctyl phosphine oxide, and cetrimonium bromide.

The first ligands preferably have a lower tendency to remain in the filmduring the formation of the semiconductor film.

Among those described above, the first ligands are preferably at leastone of oleic acid or oleylamine from the viewpoints of impartingdispersion stability to the semiconductor quantum dots and possessing alower tendency to remain in the semiconductor film.

The content of the first ligands in the semiconductor quantum dotdispersion liquid is preferably from 10 mmol/1 to 200 mmol/1 withrespect to the total volume of the semiconductor quantum dot dispersionliquid.

(First Solvent)

The first solvent contained in the semiconductor quantum dot dispersionliquid is not particularly limited. The first solvent is preferably asolvent that hardly dissolves the semiconductor quantum dots but easilydissolves the first ligands. The first solvent is preferably an organicsolvent, specific examples of which include alkanes (for example,n-hexane, n-octane, and the like), benzene, and toluene.

The first solvent may be used singly, or may be a mixed solventobtainable by mixing two or more solvents.

The first solvent is preferably a solvent having a lower tendency toremain in the semiconductor film to be formed, among the solventsdescribed above. In the case of using a solvent having a relatively lowboiling point, the content of residual organic matter in the finallyobtained semiconductor film can be regulated to be a low value.

Solvents having high wettability with respect to the substrate arenaturally preferred. For example, in the case of application to a glasssubstrate, alkanes such as hexane or octane are more preferred.

The content of the first solvent in the semiconductor quantum dotdispersion liquid is preferably from 90% by mass to 98% by mass withrespect to the total mass of the semiconductor quantum dot dispersionliquid.

—Substrate—

The semiconductor quantum dot dispersion liquid is applied to asubstrate.

The shape, structure, size, and the like of the substrate are notparticularly limited, and may be selected, as appropriate, in accordancewith the purpose. The structure of the substrate may be a single layerstructure or a multilayer structure. Examples of substrates that can beused include a substrate formed of an inorganic material such as glassor YSZ (Yttria-Stabilized Zirconia), a resin, a resin compositematerial, or the like. Among them, substrates formed of a resin or aresin composite material are preferable in consideration of the lightweight and flexibility thereof.

Examples of resins include synthetic resins such as polybutyleneterephthalate, polyethylene terephthalate, polyethylene naphthalate,polybutylene naphthalate, polystyrene, polycarbonate, polysulphone,polyether sulfone, polyarylate, allyl diglycol carbonate, polyamide,polyimide, polyamideimide, polyetherimide, polybenzazole, polyphenylenesulfide, polycycloolefin, norbornene resins, fluorine resins such aspolychlorotrifluoroethylene, liquid crystal polymers, acrylic resins,epoxy resins, silicone resins, ionomer resins, cyanate resins,cross-linked fumaric acid diester, cyclic polyolefin, aromatic ether,maleimide-olefin, celluouse, and episulfide compounds.

Examples of composite materials of an inorganic material and a resininclude a composite plastic material formed from a resin and any of thefollowing inorganic materials. Specifically, examples thereof include acomposite plastic material formed from a resin and silicon oxideparticles, a composite plastic material formed from a resin and metalnanoparticles, a composite plastic material formed from a resin andinorganic oxide nanoparticles, a composite plastic material formed froma resin and inorganic nitride nanoparticles, a composite plasticmaterial formed from a resin and carbon fibers, a composite plasticmaterial formed from a resin and carbon nanotubes, a composite plasticmaterial formed from a resin and glass flakes, a composite plasticmaterial formed from a resin and glass fibers, a composite plasticmaterial formed from a resin and glass beads, a composite plasticmaterial formed from a resin and clay mineral, a composite plasticmaterial formed from a resin and particles having a mica-derived crystalstructure, a laminated plastic material having at least one contactinterface between a resin and thin glass, and a composite materialhaving a barrier function and at least one contact interface formed byalternately stacking inorganic layers and organic layers.

It is also possible to use a stainless substrate, a metal multilayersubstrate in which stainless and another metal are disposed in layers,an aluminum substrate, an aluminum substrate which is provided with anoxide film and of which the insulating properties of the surface thereofhave been improved by applying oxidation treatment (for example,anodization treatment) to the surface, or the like.

The substrate formed from a resin or a resin composite material (a resinsubstrate or a resin composite material substrate) preferably hasexcellent properties with respect to heat resistance, dimensionalstability, solvent resistance, electrical insulating properties,workability, low air permeability, low hygroscopicity, and the like. Theresin substrate and the resin composite material substrate may have, forexample, a gas barrier layer for preventing permeation of water, oxygen,or the like, or an undercoat layer for improving the flatness of theresin substrate or the adhesion to a lower electrode.

A lower electrode, an insulating film, or the like may be provided onthe substrate. In this case, the semiconductor quantum dot dispersionliquid is applied to the lower electrode or the insulating film providedon the substrate.

Although the thickness of the substrate is not particularly limited, thethickness is preferably from 50 μm to 1000 μm, and more preferably from50 μm to 500 μm. A thickness of the substrate of 50 μm or more improvesthe flatness of the substrate itself, and a thickness of the substrateof 1000 μm or less improves the flexibility of the substrate itself, andmakes it easier to use a semiconductor film as a flexible semiconductordevice.

Methods employed for applying the semiconductor quantum dot dispersionliquid to the substrate are not particularly limited, and examplesthereof include a method of applying the semiconductor quantum dotdispersion liquid to the substrate, and a method of immersing thesubstrate in the semiconductor quantum dot dispersion liquid.

More specific examples of methods for applying the semiconductor quantumdot dispersion liquid to the substrate include liquid phase methods suchas a spin coating method, a dipping method, an ink jetting method, adispenser method, a screen printing method, a letterpress printingmethod, an intaglio printing method, and a spray coating method.

In particular, an ink jetting method, a dispenser method, a screenprinting method, a letterpress printing method, and an intaglio printingmethod enable a coating film to be formed at any position on thesubstrate, and, in addition, eliminate the necessity for a patterningprocess after film formation. Therefore, process cost can be reducedwhen using these methods.

Ligand Exchange Process

In the ligand exchange process, a solution that includes a secondsolvent and a second ligands (specific amine-based ligands) having ashorter molecular chain length than that of the first ligands and beingat least one kind selected from the group consisting of a ligandrepresented by Formula (A), a ligand represented by Formula (B), and aligand represented by Formula (C), is applied to the cluster ofsemiconductor quantum dots that has been formed on the substrate by thesemiconductor quantum dot cluster formation process, whereby the firstligands coordinating to respective semiconductor quantum dots arereplaced by the second ligands contained in the ligand solution.

—Ligand Solution—

The ligand solution contains at least the second ligands (specificamine-based ligands) and the second solvent. The ligand solution mayfurther contain other components as far as the effects of the inventionare not impaired.

(Second Ligands)

The second ligands are the specific amine-based ligands described above,which are at least one kind of ligand having a shorter molecular chainlength than that of the first ligands and being selected from the groupconsisting of a ligand represented by Formula (A), a ligand representedby Formula (B), and a ligand represented by Formula (C).

The method employed for determining whether the length of the molecularchains of the ligands is shorter or longer is the same as that describedin the explanation of the first ligands.

When A¹ or B¹ in Formula (A), A² or B² in Formula (B), or A³ in Formula(C) represents a substituent having from 1 to 10 atoms, examples of thesubstituent having from 1 to 10 atoms include alkyl groups having from 1to 3 carbon atoms (a methyl group, an ethyl group, a propyl group, andan isopropyl group), alkenyl groups having 2 to 3 carbon atoms (anethenyl group and a propenyl group), alkynyl groups having from 2 to 4carbon atoms (an ethynyl group, a propynyl group, and the like), acyclopropyl group, alkoxy groups having from 1 to 2 carbon atoms (amethoxy group and an ethoxy group), acyl groups having from 2 to 3carbon atoms (an acetyl group and a propionyl group), alkoxycarbonylgroups having from 2 to 3 carbon atoms (a methoxycarbonyl group and anethoxycarbonyl group), acyloxy groups having 2 carbon atoms (anacetyloxy group), acylamino groups having 2 carbon atoms (an acetylaminogroup), hydroxyalkyl groups having from 1 to 3 carbon atoms (ahydroxymethyl group, a hydroxyethyl group, and a hydroxypropyl group),an aldehyde group (—COH), a hydroxy group (—OH), a carboxy group(—COOH), a sulfo group (—SO₃H), a phospho group (—OPO(OH)₂), an aminogroup (—NH₂), a carbamoyl group (—CONH₂), a cyano group (—CN), anisocyanate group (—N═C═O), a thiol group (—SH), a nitro group (—NO₂), anitroxy group (—ONO₂), an isothiocyanate group (—NCS), a cyanate group(—OCN), a thiocyanate group (—SCN), an acetoxy group (OCOCH₃), anacetamide group (NHCOCH₃), a formyl group (—CHO), a formyloxy group(—OCHO), a formamide group (—NHCHO), a sulfamino group (—NHSO₃H), asulfino group (—SO₂H), a sulfamoyl group (—SO₂NH₂), a phosphono group(—PO₃H₂), an acetyl group (—COCH₃), halogen atoms (a fluorine atom, achlorine atom, a bromine atom, and the like), and alkali metal atoms (alithium atom, a sodium atom, a potassium, and the like).

As far as the total number of atoms in the substituent is 10 or fewer,the substituent may itself has a further substituent.

When the number of atoms in the substituent is 10 or fewer, sterichindrance due to the presence of ligands is reduced, and thesemiconductor quantum dots can come closer to each other; therefore, theelectrical conductivity of the semiconductor film can be enhanced.

It is preferable that the substituent has 7 or fewer atoms from theviewpoint of further reducing the spacing between the semiconductorquantum dots, and it is more preferable that the substituent is ahydrogen atom.

X¹ in Formula (A) and X² in Formula (B) are preferably —OH (a hydroxygroup) in consideration of the solubility when an alcohol solution ofthe specific amine-based ligands is to be prepared.

Specific examples of the compound represented by Formula (A) include2-aminoethanol, 2-aminoethane-1-thiol, 1-amino-2-butanol,1-amino-2-pentanol, L-cystine, and D-cystine. Specific examples of thecompound represented by Formula (B) include 3-amino-1-propanol,L-homoserine, and D-homoserine. Specific examples of the compoundrepresented by Formula (C) include aminohydroxyacetic acid.

The specific amine-based ligands may be a derivative of a compoundrepresented by Formula (A), a derivative of a compound represented byFormula (B), or a derivative of a compound represented by Formula (C),such as 2-aminoethanethiol derivative, 2-aminoethanol derivative, or3-amino-1-propanol derivative.

Using any of the above compounds as the specific amine-based ligandsenables the inter-dot distance to be regulated to less than 0.45 nm, anda high photocurrent value can be obtained as compared to a case in whichethanedithiol is used as ligands. In particular, using 2-aminoethanol or2-aminoetahne-1-thiol, which is represented by Formula (A), as ligandsproduces a large effect with respect to an increase in the photocurrentvalue.

It is conceivable that the reasons therefor are the following tworeasons. Specifically, a dangling bond of a metal atom in eachsemiconductor quantum dot, —NH₂ illustrated in Formula (A), and SH (orOH) illustrated as X¹ in Formula (A) form a five-membered cyclicchelate, and the formation of the five-membered cyclic chelate makes iteasier to obtain a high complex stability constant (log 62). Further,the chelate coordination of the specific amine-based ligands to themetal atom in each semiconductor quantum dot reduces the sterichindrance between semiconductor quantum dots, and, consequently, a highelectrical conductivity can easily be obtained. The coordinationmechanism as described above applies to the case of a derivative of a2-aminoethanol or a derivative of 2-aminoethanethiol. Therefore, a highphotocurrent value can be obtained, and a high electrical conductivitycan be obtained, even in the case of using a derivative of2-aminoethanol or a derivative of 2-aminoethanethiol.

When an alcohol is used as the second solvent contained in the ligandsolution, the specific amine-based ligands preferably have a hydroxygroup (OH) in a molecule thereof. When the specific amine-based ligandshave a hydroxy group in the molecular structure thereof, miscibilitythereof with alcohol can be improved, and efficient ligand exchange canbe achieved.

The content of the specific amine-based ligands in the ligand solutionis preferably from 5 mmol/l to 200 mmol/l, and more preferably from 10mmol/l to 100 mmol/l, with respect to the total volume of the ligandsolution.

(Second Solvent)

The second solvent contained in the ligand solution is not particularlylimited. The second solvent is preferably a solvent that has highability to dissolve the specific amine-based ligands.

Organic solvents having a high dielectric constant are preferable assuch a solvent, and examples thereof include ethanol, acetone, methanol,acetonitrile, dimethylformamide, dimethylsulfoxide, butanol, andpropanol.

The second solvent may be used singly, or may be a mixed solventobtainable by mixing two or more solvents.

Among the solvents described above, the second solvent is preferably asolvent having a lower tendency to remain in the semiconductor film tobe formed. In consideration of the ease of drying and ease of removal bywashing, alcohols having a low boiling point or alkanes are preferable,and methanol, ethanol, n-hexane, or n-octane is more preferable.

It is preferable that the second solvent does not mix with the firstsolvent. For example, when an alkane such as hexane or octane is used asthe first solvent, it is preferable to use a polar solvent such asmethanol or acetone as the second solvent.

The content of the second solvent in the ligand solution is theremaining part left after subtracting the content of the specificamine-based ligands from the total mass of the ligand solution.

Methods that can be employed for applying the ligand solution to thecluster of semiconductor quantum dots are the same as the methods thatcan be employed for applying the semiconductor quantum dot dispersionliquid to the substrate, and preferred embodiments thereof are also thesame.

The semiconductor quantum dot cluster formation process and the ligandexchange process may be performed repeatedly. Repeatedly performing thesemiconductor quantum dot cluster formation process and the ligandexchange process can increase the electrical conductance of thesemiconductor film having the cluster of semiconductor quantum dots towhich the specific amine-based ligands coordinate, and enables thethickness of the semiconductor film to be increased.

Repeatedly performing the semiconductor quantum dot cluster formationprocess and the ligand exchange process may include performing therespective processes separately and independently; however, repeatedlyperforming the semiconductor quantum dot cluster formation process andthe ligand exchange process preferably includes repeating a cycle inwhich the ligand exchange process is performed after the semiconductorquantum dot cluster formation process is performed. Repeating thesemiconductor quantum dot cluster formation process and the ligandexchange process in sets makes it easier to suppress non-uniformity ofligand exchange.

In the case of repeatedly performing the semiconductor quantum dotcluster formation process and the ligand exchange process, it ispreferable to perform sufficient drying of the film every cycle.

It is conceivable that the higher the ratio of replacement with thespecific amine-based ligands during the ligand exchanging in the clusterof semiconductor quantum dots is, the higher the photocurrent value ofthe semiconductor film becomes.

Here, the ligand exchange between the first ligands and the secondligands (the specific amine-based ligands) in the semiconductor quantumdots may include ligand exchange in at least a portion of the cluster ofsemiconductor quantum dots, and it is not essential that 100% (innumber) replacement by the specific amine-based ligands occur.

The method of manufacturing a semiconductor film according to thepresent embodiment may further include, for example, one or moreselected from a dispersion liquid drying process of drying thesemiconductor quantum dot dispersion liquid, a solution drying processof drying the ligand solution, or a washing process of washing thecluster of semiconductor quantum dots provided on the substrate.

Washing Process

The method of manufacturing a semiconductor film according to theinvention may further include a washing process of washing the clusterof semiconductor quantum dots provided on the substrate.

When the washing process is included, excess ligands and ligandsdetached from the semiconductor quantum dots can be removed, andresidual solvents and other impurities can also be removed. The washingof the cluster of semiconductor quantum dots may be performed by pouringat least one of the first solvent or the second solvent onto the clusterof semiconductor quantum dots, or by immersing a substrate havingthereon the cluster of semiconductor quantum dots or the semiconductorfilm in at least one of the first solvent or the second solvent.

The washing by the washing process may be performed after thesemiconductor quantum dot cluster formation process or after the ligandexchange process. The washing may be performed after the repetition of aset of processes composed of the semiconductor quantum dot clusterformation process and the ligand exchange process.

Drying Process

The method of manufacturing a semiconductor film according to theinvention may include a drying process.

The drying process may be a dispersion liquid drying process of drying asolvent remaining in the cluster of semiconductor quantum dots after thesemiconductor quantum dot cluster formation process, or a solutiondrying process of drying a ligand solution after the ligand exchangeprocess. Further, the drying process may be a process of drying in alump performed after the repetition of the set of processes composed ofthe semiconductor quantum dot cluster formation process and the ligandexchange process.

A semiconductor film is manufactured on a substrate through theprocesses described above.

In the semiconductor film obtained, semiconductor quantum dots areconnected to each other with a specific amine-based ligand therebetween,the specific amine-based ligand being shorter than that of conventionalart. As a result, the inter-dot distance in the semiconductor film isless than 0.45 nm, and the semiconductor film has a high electricalconductivity, whereby a high photocurrent value can be obtained. Sincethe complex stability constant of the specific amine-based ligand ishigh, the coordination bonds in the semiconductor film according to theinvention formed from the semiconductor quantum dots and the specificamine-based ligands are stable, and the semiconductor film has excellentfilm strength and suppressed film detachment.

It is conceivable that the spacing between semiconductor quantum dots inthe semiconductor film manufactured according to the present embodimentis short due to the formation of coordination bonding between thesemiconductor quantum dots and the specific amine-based ligands havingonly a small number of atoms and represented by Formula (A), (B), or(C). It is conceivable that, as a result of the short spacing, thesemiconductor quantum dots are arranged densely, and the overlapping ofwave functions between semiconductor quantum dots can increase. It isconceivable that, as a result of this, the electrical conductivityincreases, and the photocurrent value increases.

The specific amine-based ligands include, in its molecule: at least oneamino group (NH₂); and —SH, —NH₂, or —OH represented by X¹ (or X²) or OHof the carboxy group. An amino group has a high complex stabilityconstant, and it is conceivable that an amino group promotes formationof a complex between the metal atom of each semiconductor quantum dotand —SH, —NH₂, or —OH represented by X¹ (or X²) or OH of the carboxygroup. It is conceivable that the promoted complex formation makesstronger the bonding between semiconductor quantum dots and the specificamine-based ligands, and, therefore, suppresses the detachment of thesemiconductor film that includes the semiconductor quantum dots and thespecific amine-based ligands.

It is conceivable that, for the reasons discussed above, using thesemiconductor film according to the invention enables obtainment of ahigh photocurrent value and suppression of film detachment.

In the semiconductor film of the present embodiment, the complexstability constant log β₁ between the specific amine-based ligand andthe metal atom of each semiconductor quantum dot is preferably 8 orgreater.

Here, the complex stability constant is determined by the relationshipbetween the ligand and the metal atom with which the ligand forms acoordination bond, and the complex stability constant is represented bythe following Formula (b).

$\begin{matrix}{{\log \; \beta_{1}} = \frac{\lbrack{ML}\rbrack}{\lbrack M\rbrack \lbrack L\rbrack}} & {{Formula}\mspace{14mu} (b)}\end{matrix}$

In Formula (b), [ML] represents the molar concentration of the complexformed by the bonding between the metal atom and the ligand, [M]represents the molar concentration of metal atoms that can participatein the coordination bonding, and [L] represents the molar concentrationof the ligand.

There are actually cases where plural ligands coordinate to one metalatom. Nevertheless, in the present embodiment, the complex stabilityconstant log β₁ represented by Formula (b) corresponding to a case whereone ligand molecule coordinates to one metal atom is determined as anindex of the strength of coordination bonding.

When the complex stability constant log β₁ between the specificamine-based ligand and the metal atom of each semiconductor quantum dotis 8 or greater, a complex easily forms.

A higher complex stability constant log β₁ of the combination of thesemiconductor quantum dot and the ligand is more preferable. Further,the bonding strength increases when the ligand is capable ofmultidentate coordination, as in the case of chelates. In general, astronger coordination bonding enables conventional long molecular chainligands to be more efficiently replaced, and enables a higher electricalconductivity to be more easily obtained. The value of the complexstability constant log β₁ of the specific amine-based ligand variesdepending on what semiconductor quantum dot material is used forconstituting the semiconductor quantum dots. Nonetheless, the specificamine-based ligand can be applied to various semiconductor quantum dotmaterials since the specific amine-based ligand has a short molecularchain length and can easily coordinate.

The log β₁ is more preferably 8 or more, and still more preferably 10 ormore.

The complex stability constant log β₁ between the specific amine-basedligand and the metal atom of each semiconductor quantum dot in thesemiconductor film of the present embodiment may be obtained using, forexample, spectroscopy, magnetic resonance spectroscopy, potentiometry,solubility measurement, chromatography, calorimetry, freezing pointmeasurement, vapor pressure measurement, relaxation measurement,viscosity measurement, or surface tension measurement.

In the present embodiment, the complex stability constant is obtainedusing Sc-Database ver. 5.85 (Academic Software) (2010) in which variousmethods and results reported from research institutions are collected.In a case in which a log β₁ value is not included in Sc-Database ver.5.85, a value described in “Critical Stability Constants” (A. E. Martelland R. M. Smith) is used. In a case in which the log β₁ value is notdescribed in “Critical Stability Constants” either, the log β₁ value iscalculated using the measurement method described above or using aprogram PKAS method for calculating the complex stability constant (A.E. Martell, et al., The Determination and Use of Stability Constants,VCH (1988)).

(2) Second Embodiment

The semiconductor film according to the invention can also be obtainedby adding, for example, a ligand agent that includes at least a thiocyangroup and a metal ion (also referred to as a specific thiocyan-basedligand agent) to the cluster of semiconductor quantum dots. The ligandagent is a compound having a ligand. In the case of using the specificthiocyan-based ligand agent, at least a thiocyan group serves as aligand and coordinates to a semiconductor quantum dot. The metal ion mayadditionally coordinate to a semiconductor quantum dot.

It is thought that, in general organic ligands (for example,ethanedithiol), a ligand group (such as SH, NH₂, and OH) coordinates toonly a cation portion of a quantum dot surface. It is presumed thatusing such general organic ligands results in an increased amount ofdangling bonds as compared to the case of using the specific ligandagent containing a thiocyan group, since the ligand groups in thegeneral organic ligands have a longer molecular chain length than thatof a thiocyan group.

Meanwhile, the specific thiocyan-based ligand agent has at least athiocyan group and a metal atom.

Examples of general ligands having a thiocyan group includetetrabutylammonium thiocyanate (TBAT). When TBAT is used as a ligandagent, a sufficient electrical conductivity is not obtained. The reasontherefor is presumably that the molecular chain of TBAT is long and thetetrabutylammonium ion portion having a large molecular weight remainsin the semiconductor film, as a result of which the electricalconduction through the semiconductor quantum dots is inhibited.

As described above, a thiocyan group has a short molecular chain lengthand has an S atom and an N atom that easily form a coordination bondwith semiconductor quantum dots; it is conceivable that the thiocyangroup therefore firmly coordinates to a semiconductor quantum dot anddecreases the distance between particles, thereby enhancing the strengthof the semiconductor film and suppressing film detachment of thesemiconductor film.

Thiocyan Group and Metal Ion (Specific Thiocyan-Based Ligand Agent)

The semiconductor film of the present embodiment includes a thiocyangroup and a metal ion.

The origins of the thiocyan group and the metal ion constituting thesemiconductor of the present embodiment are not particularly limited.The metal ion may be a monovalent metal ion, or a di- or higher-valentmetal ion. Further, the metal ion may be an alkali metal ion, analkaline earth metal ion, or a transition metal ion. Among them, themetal ion is preferably an alkali metal ion, and the metal ion ispreferably a potassium ion or a lithium ion.

The semiconductor film of the present embodiment may include only onekind of metal ion or a mixture of two or more kinds of metal ions.

The semiconductor film of the present embodiment can be obtained byadding, for example, a ligand agent that includes at least a thiocyangroup and a metal ion (specific thiocyan-based ligand agent) to thecluster of semiconductor quantum dots.

Examples of the ligand agent (specific thiocyan-based ligand agent)containing at least a thiocyan group and a metal ion include potassiumthiocyanate, barium thiocyanate, mercury bisthiocyanate, calciumthiocyanate, cadmium thiocyanate, copper thiocyanate, lithiumthiocyanate, silver thiocyanate, cobalt thiocyanate, leadbisthiocyanate, nickel thiocyanate, sodium thiocyanate, zincthiocyanate, thallium thiocyanate, strontium thiocyanate, sodiumtris(thiocyanate), iron bis(thiocyanate), iron tris(thiocyanate),manganese bisthiocyanate, oxozirconium bis(thiocyanate), and oxohafniumbis(thiocyanate).

The semiconductor film of the present embodiment includes at least thecluster of the semiconductor quantum dots containing the metal atom, athiocyan group, and a metal ion, and at least the thiocyan groupcoordinates to the semiconductor quantum dots. Due to the semiconductorfilm having such a configuration, a high photocurrent value can beobtained, and film detachment is suppressed.

The S atom and the N atom in a thiocyan group that coordinates to asemiconductor quantum dot have a high tendency to form a coordinationbond with a cation portion of the semiconductor quantum dot, and, at thesame time, the metal atom has a high tendency to form a coordinationbond with an anion portion of the semiconductor quantum dot. It isconceivable that dangling bond of both the cation portion and the anionportion is consequently reduced, and, due to the reduction in defects,overlapping of wave functions between semiconductor quantum dots can beenhanced. It is conceivable that a high electrical conductivity canresultantly be obtained.

The method of manufacturing a semiconductor film according to thepresent embodiment using the specific thiocyan-based ligand agentincludes:

a semiconductor quantum dot cluster formation process of applying asemiconductor quantum dot dispersion liquid, to form a cluster ofsemiconductor quantum dots, the semiconductor quantum dot dispersionliquid including semiconductor quantum dots containing a metal atom,first ligands coordinating to respective semiconductor quantum dots, anda first solvent; and

a ligand exchange process of applying, to the cluster, a ligand agentsolution and replacing the first ligands, which coordinate to thesemiconductor quantum dots, by a second ligand agent, the ligand agentsolution including the second ligand agent (specific thiocyan-basedligand agent) and a second solvent, and the second ligand agent beingshorter than the first ligand and including a thiocyan group and a metalion.

Semiconductor Quantum Dot Cluster Formation Process

The semiconductor quantum dot cluster formation process is the same asthat in the first embodiment, and description thereof is omitted.

The second ligand agent (specific thiocyan-based ligand agent) is acompound having a thiocyan group and a metal ion, as described above.When the second ligand agent has coordinated to a semiconductor quantumdot, the second ligand agent is difficult to disperse in organicsolvents.

Ligand Exchange Process

In the ligand exchange process, a ligand agent solution that includes asecond solvent and the second ligand agent (specific thiocyan-basedligand agent) having a shorter molecular chain length than that of thefirst ligands and having a thiocyan group and a metal ion is applied tothe cluster of semiconductor quantum dots that has been formed on thesubstrate through the semiconductor quantum dot cluster formationprocess, and the first ligands coordinating to respective semiconductorquantum dots are replaced by the second ligands contained in the ligandagent solution.

—Ligand Agent Solution—

The ligand agent solution includes at least the second ligand agent(specific thiocyan-based ligand agent) and the second solvent.

The ligand agent solution may further include other components as far asthe effects of the invention are not impaired.

(Second Ligand Agent)

The second ligand agent is the specific thiocyan-based ligand agentdescribed above, and has a molecular chain length shorter than that ofthe first ligands. The method employed for determining whether or notthe length of the molecular chains of the ligands is longer or shorteris the same as that described in the description of the first ligands inthe first embodiment, in which the specific amine-based ligands areused.

The details of the specific thiocyan-based ligand agent are also thesame as those described above.

The content of the specific thiocyan-based ligand agent in the ligandagent solution is preferably from 5 mmol/l to 200 mmol/l, and morepreferably from 10 mmol/l to 100 mmol/l, with respect to the totalvolume of the ligand agent solution.

(Second Solvent)

The second solvent contained in the ligand agent solution is notparticularly limited, and the second solvent is preferably a solventthat easily dissolves the specific thiocyan-based ligand agent.

Organic solvents having a high dielectric constant are preferable assuch a solvent, and examples thereof include ethanol, acetone, methanol,acetonitrile, dimethylformamide, dimethylsulfoxide, butanol, andpropanol.

The second solvent may be used singly, or may be a mixed solventobtainable by mixing two or more solvents.

The second solvent is preferably a solvent that has a lower tendency toremain in the formed semiconductor film, among the solvents describedabove. From the viewpoints of ease of drying and ease of removal bywashing, alcohol having low boiling points and alkanes are preferable,and methanol, ethanol, n-hexane, or n-octane is more preferable.

It is preferable that the second solvent does not mix with the firstsolvent. For example, when an alkane such as hexane or octane is used asthe first solvent, a polar solvent such as methanol or acetone ispreferably used as the second solvent.

The content of the second solvent in the ligand agent solution is theremaining part left after subtracting the content of the specificthiocyan-based ligand agent from the total mass of the ligand agentsolution.

Methods that can be employed for applying the ligand agent solution tothe cluster of semiconductor quantum dots are the same as the methodsthat can be employed for applying the semiconductor quantum dotdispersion liquid onto the substrate, and preferred embodiments thereofare also the same.

When the first ligands are replaced by the second ligand agent (specificthiocyan-based ligand agent), at least a thocyan group from among theconstituent elements of the specific thiocyan-based ligand agentcoordinates to the metal atom of a semiconductor quantum dot.Specifically, coordination to the metal atom of the semiconductorquantum dot occurs via at least one of the S atom or the N atom of thethiocyan group consisting of three atoms. It is conceivable that thethiocyan-based ligand agent, in which the size of the ligand is as smallas three atoms, has a high ability to diffuse into the semiconductorfilm as compared with the case of coordination of ligands having a longmolecular chain, thereby enabling efficient ligand exchange. The metalion therein may coordinate to the metal ion of a semiconductor quantumdot, may be diffusely present as a counter ion for the thiocyan grouprather than performing coordination, or may be present as a free ion.

Through the ligand exchange process, the first ligands coordinating tothe semiconductor quantum dots and having a molecular chain lengthlonger than that of the second ligand agent is replaced by the secondligand agent by applying the solution containing the specificthiocyan-based ligand agent to the cluster of semiconductor quantumdots. Here, the second ligand agent contains at least a thiocyan groupand a metal ion, and is the specific thiocyan-based ligand agentdescribed above.

As a result of the ligand exchange, the thiocyan group in the specificthiocyan-based ligand agent coordinates to at least the metal atom of asemiconductor quantum dot.

It is conceivable that, through the ligand exchange process, thethiocyan group in the second ligand agent (specific thiocyan-basedligand agent) having a molecular chain length shorter than that of thefirst ligands coordinates to each semiconductor quantum dot, instead ofthe first ligands, to form a coordination bond with the semiconductorquantum dot, as a result of which the semiconductor quantum dots caneasily come close to each other. Due to the semiconductor quantum dotscoming closer to each other, the inter-dot distance is regulated to beless than 0.45 nm, and the electrical conductivity of the cluster ofsemiconductor quantum dots is enhanced, whereby a semiconductor filmhaving a high photocurrent value can be obtained.

Also in the present embodiment, the semiconductor quantum dot clusterformation process and the ligand exchange process may repeatedly beperformed. Repeatedly performing the semiconductor quantum dot clusterformation process and the ligand exchange process enables thesemiconductor film having the cluster of semiconductor quantum dotscoordinated with the specific thiocyan-based ligand agent to have anincreased electric conductivity and also enables the thickness of thesemiconductor film to be increased.

Repeatedly performing the semiconductor quantum dot cluster formationprocess and the ligand exchange process may include performing therespective processes separately and independently, but preferablyincludes repeatedly performing a cycle in which the ligand exchangeprocess is performed after the semiconductor quantum dot clusterformation process is performed. Repeatedly performing the set ofprocesses composed of the semiconductor quantum dot cluster formationprocess and the ligand exchange process makes it easier to suppressnon-uniformity of ligand exchange.

In the case of repeatedly performing the semiconductor quantum dotcluster formation process and the ligand exchange process, it ispreferable to perform sufficient drying of the film every cycle.

It is conceivable that the higher the ratio of replacement with thespecific thiocyan-based ligand agent during the ligand exchanging in thecluster of semiconductor quantum dots is, the higher the photocurrentvalue of the semiconductor film becomes.

Here, the ligand exchange between the first ligands and the secondligands (the specific thiocyan-based ligand agent) in the semiconductorquantum dots may include ligand exchange in at least a portion of thecluster of semiconductor quantum dots, and it is not essential that 100%(in number) replacement by the specific thiocyan-based ligand agentoccur.

<Electronic Device>

The uses of the semiconductor film according to the invention are notlimited. Since the semiconductor film according to the invention hasphotoelectric conversion characteristics and the detachment thereof issuppressed, the semiconductor film according to the invention cansuitably be applied to various electronic devices having a semiconductorfilm or a photoelectric conversion film.

Specifically, the semiconductor film according to the invention cansuitably be applied to a photoelectric conversion film of a solar cell,a light-emitting diode (LED), a semiconductor layer (active layer) of athin film transistor, a photoelectric conversion film of an indirectradiation imaging apparatus, a visible to infrared photodetector, andthe like.

<Solar Cell>

A solar cell is described below as one example of an electronic devicethat includes the semiconductor film according to the invention or thesemiconductor film manufactured by the method of manufacturing asemiconductor film according to the invention.

For example, a p-n junction solar cell can be configured using asemiconductor film device that has a p-n junction and that includes ap-type semiconductor layer including the semiconductor film according tothe invention and an n-type semiconductor layer.

Examples of more specific embodiments of the p-n junction solar cellinclude an embodiment in which a p-type semiconductor layer and ann-type semiconductor layer are provided adjacent to each other on atransparent conductive film formed on a transparent substrate, and inwhich a metal electrode is formed on the p-type semiconductor layer andthe n-type semiconductor layer.

One example of the p-n junction solar cell is described below withreference to FIG. 1.

FIG. 1 is a schematic cross-sectional view of a p-n junction solar cell100 according to an embodiment of the invention. The p-n junction solarcell 100 has a layered structure that includes a transparent substrate10, a transparent conductive film 12 provided on the transparentsubstrate 10, a p-type semiconductor layer 14 provided on thetransparent conductive film 12 and configured by the semiconductor filmaccording to the invention, an n-type semiconductor layer 16 provided onthe p-type semiconductor layer 14, and a metal electrode 18 provided onthe n-type semiconductor layer 16.

The p-n junction solar cell can be obtained due to the p-typesemiconductor layer 14 and the n-type semiconductor layer 16 disposed asadjacent layers.

Examples of materials that can be used as the transparent substrate 10are the same as those of the materials that can be used as the substratefor use in the method of manufacturing a semiconductor film according tothe invention, provided that the substrate is transparent. Specificexamples thereof include glass substrates and resin substrates.

Examples of the transparent conductive film 12 include a film formedfrom a In₂O₃:Sn (ITO), SnO₂:Sb, SnO₂:F, ZnO:Al, ZnO:F, CdSnO₄, or thelike.

As the p-type semiconductor layer 14, the semiconductor film accordingto the invention is used, as described above,

The n-type semiconductor layer 16 is preferably a metal oxide. Specificexamples thereof include a metal oxide containing at least one of Ti,Zn, Sn, or In, and more specific examples include TiO₂, ZnO, SnO₂, andIGZO. From the viewpoint of manufacturing cost, the n-type semiconductorlayer is preferably formed using a wet process (also referred to as aliquid phase method), similar to the case of the p-type semiconductorlayer.

As the metal electrode 18, Pt, Al, Cu, Ti, Ni, or the like may be used.

EXAMPLES

Examples are described below. However, the examples should not beconstrued as limiting the invention.

Preparation of Semiconductor Quantum Dot Dispersion Liquid 1

First, a PbS particle dispersion liquid in which PbS particles aredispersed in toluene was prepared. The PbS particle dispersion liquidused was PbS CORE EVIDOT (nominal particle diameter: 3.3 nm,concentration: 20 mg/ml, solvent: toluene) manufactured by EvidentTechnologies, Inc.

Subsequently, 2 ml of the PbS particle dispersion liquid was added intoa centrifuge tube, 38 μl of oleic acid was added thereto, and 20 ml oftoluene was further added thereto, thereby decreasing the concentrationof the dispersion liquid. Thereafter, the PbS particle dispersion liquidwas subject to ultrasonic dispersing treatment, and the PbS particledispersion liquid was agitated well. Then, 40 ml of ethanol was added tothe PbS particle dispersion liquid, and ultrasonic dispersing treatmentwas further carried out, and centrifugal separation was carried out at10000 rpm and 3° C. for 10 minutes. The supernatant in the centrifugetube was discarded, 20 ml of octane was added into the centrifuge tube,and ultrasonic dispersing treatment was performed, thereby dispersingprecipitated quantum dots in the octane solvent well. The resultantdispersion was subjected to concentration using a rotary evaporator (35hpa, 40° C.), as a result of which about 4 ml of semiconductor quantumdot dispersion liquid 1 (solvent: octane) having a concentration ofabout 10 mg/ml was obtained.

The particle diameters of the PbS particles contained in semiconductorquantum dot dispersion liquid 1 were measured using STEM (scanningtransmission electron microscope), and analyzed with an image checkingsoftware, whereby the average particle diameter of the PbS particlescontained in the semiconductor quantum dot dispersion liquid 1 was foundto be 3.2 nm.

Preparation of Semiconductor Quantum Dot Dispersion Liquid 2

30 ml of 1-octadecene, 6.32 mmol of lead oxide (II), and 21.2 mmol ofoleic acid were individually weighed, and mixed in a three-necked flask.The mixture was stirred at 300 rpm using a magnetic stirrer of analuminum block hot plate stirrer. During stirring of the mixture, themixture was deaerated and dehydrated at 120° C. for 1 hour under reducedpressure. Subsequently, the three-necked flask was cooled to roomtemperature using a cooling fan, and ventilation with a nitrogen gas wascarried out. Then, a solution containing 2.57 mmol ofhexamethyldisilathiane and 5 ml of 1-octadecene was injected into thethree-necked flask using a syringe (allowing a syringe needle topenetrate through a septum cap). Thereafter, the three-necked flask washeated to 120° C. over 40 minutes, and maintained at that temperaturefor 1 minute. Then, the three-necked flask was cooled to the roomtemperature using a cooling fan. Undesired matter was removed from theresultant product using a centrifuge, thereby separating only PbSparticles. The particles were dispersed in octane. When the undesiredmatter was removed from the product, toluene was used as a good solvent,and dehydrated ethanol was used as a poor solvent.

As a result of observation under a STEM, it was found that the averageparticle diameter of the obtained PbS was 5 nm.

This octane dispersion liquid of PbS quantum dots was diluted withhexane solvent to obtain semiconductor quantum dot dispersion liquid 2(a mixed solvent of octane and hexane in a volume ratio of 1:9(octane:hexane)) having a concentration of about 10 mg/ml.

Preparation of Ligand Solution

1 mmol of the ligand indicated in the column “ligand” in Table 1 wasprepared, and dissolved in 10 ml of methanol, to prepare a ligandsolution having a concentration of 0.1 mol/1. Ultrasonic waves areapplied thereto in order to promote the dissolution of the ligand in theligand solution, thereby avoiding occurrence of residual undissolvedligand as much as possible.

<Measurement of Average Shortest Inter-Dot Distance BetweenSemiconductor Quantum Dots in Semiconductor Film>

First, quantum dot films (samples) of Examples 1 to 3 and ComparativeExamples 1 and 2 were manufactured as follows.

Preparation of Semiconductor Film

First, a hexamethyldisilazane solution was spin-coated on quartz glass,thereby hydrophobizing the surface. Thereafter, a semiconductor filmhaving a cluster of semiconductor quantum dots was prepared according tothe following procedures.

(I) Semiconductor Quantum Dot Cluster Formation Process

Semiconductor quantum dot dispersion liquid 2 prepared was drop-castedon a substrate, to obtain a semiconductor quantum dot cluster film.

(II) Ligand Exchange Process

Further, the semiconductor quantum dot cluster film was immersed in amethanol solution of the ligand indicated in Table 1 for 3 minutes,thereby performing treatment for replacing oleic acid as the firstligand by the ligand indicated in Table 1. In this manner, the quantumdot films of Examples and Comparative Examples were obtained.

(III) Washing Process I

Subsequently, each quantum dot film was immersed in methanol solvent.

(IV) Washing Process II

Further, each quantum dot film after washing in the washing process iwas immersed in octane solvent.

A series of processes (I) to (IV) was repeated for two cycles, to obtaina ligand-exchanged semiconductor film formed of a cluster of PbS quantumdots and having a thickness of 20 nm.

In Comparative Example 2, the semiconductor quantum dot cluster film atthe time of completion of (I) semiconductor quantum dot clusterformation process was used.

The quantum dot films (samples) of Examples 1 to 3 and ComparativeExamples 1 and 2 obtained were subjected to structure evaluation using agrazing incidence small angle X-ray scattering method (GISAXS). A Kαline of Cu was used as incident light, the quantum dot film wasirradiated with the X rays at an incident angle (approximately 0.4°)that is slightly larger than the critical angle for total reflection,and scattered light was detected by moving a detector in a scanningmanner in an in-plane direction.

The in-plane angle dependence of the scattered light left aftersubtracting a scattered light background structure for the respectiveligand species is indicated in FIG. 2F, with respect to each of thesemiconductor films of Examples 1 to 3 and Comparative Example 1.

Scatter peaks that reflect the in-plane periodic structure were obtainedfor all of the samples, as illustrated in FIG. 2 for Examples 1 to 3 andComparative Example 1, and also for Comparative Example 2 although notillustrated in the drawings. When a scatter peak position obtained isrepresented by θ_(MAX), the center-to-center distance d betweensemiconductor quantum dots in the sample is calculated based on thefollowing Equation (D):

d=λ/2 sin θ_(MAX)   (D)

In Equation (D), λ represents the wavelength of the incident light.

Table 1 indicates the shortest of the distances between adjacent quantumdots calculated from the scattering peaks (after subtraction of theparticle diameter of the quantum dot from the measured center-to-centerdistance d between the quantum dots). The detected scattered light wasan average of scattered X rays from all of the regions of the samplethat were irradiated with X rays in the measuring instrument.

TABLE 1 Distance between Ligand quantum dots Example 12-aminoethane-1-thiol 0.14 nm Example 2 2-aminoethanol 0.29 nm Example 3potassium thiocyanate 0.42 nm Comparative ethanedithiol 0.45 nm Example1 Comparative oleic acid (untreated) 1.31 nm Example 2

From Table 1, it is understood that the spacing between quantum dots(distance between quantum dots) was decreased in each of the films inwhich oleic acid as initial ligands (first ligands) were subjected tothe ligand exchange treatment. In particular, in the films in which theligands were replaced by 2-aminoethane-1-thiol, 2-aminoethanol, orpotassium thiocyanate, the inter-dot distance was less than 0.45 nm, andit was confirmed that the proximity between semiconductor quantum dotsin these films was even closer than that in conventional ethanedithiolcoordination films.

<Evaluation of Electrical Conduction Property and Photocurrent Value>

Manufacture of Semiconductor Film

As a substrate on which a semiconductor film formed of a cluster ofsemiconductor dot particles is to be formed, a substrate having 65 pairsof comb-shaped platinum electrodes illustrated in FIG. 3 on quartz glasswas prepared. The comb-shaped platinum electrodes used were comb-shapedelectrodes manufactured by BAS Inc. (model number: 012126, distancebetween electrodes: 5 μm). A semiconductor film was formed thereonaccording to the following operations.

(1) Semiconductor Quantum Dot Cluster Formation Process

Semiconductor quantum dot dispersion liquid 1 prepared was dropped onthe substrate, and, thereafter, spin-coated at 2500 rpm, as a result ofwhich a semiconductor quantum dot cluster film was obtained.

(2) Ligand Exchange Process

Further, a methanol solution of the ligand indicated in Table 2 (ligandsolution) was dropped on the semiconductor quantum dot cluster film,and, thereafter, spin-coated at 2500 rpm, as a result of which asemiconductor film was obtained.

(3) Washing Process 1

Subsequently, methanol, which is the solvent of the ligand solution, wasdropped alone on the semiconductor film, and spin-coated.

(4) Washing Process 2

Further, octane solvent was dropped alone on the semiconductor filmafter washing in the washing process 1, and the octane solvent wasspin-coated.

A series of processes (1) to (4) was repeated for 15 cycles, to obtain aligand-exchanged semiconductor film formed of a cluster of PbS quantumdots and having a thickness of 100 nm.

In this manner, a semiconductor film device having a semiconductor filmon a substrate was manufactured.

The semiconductor film of the obtained semiconductor film device wassubjected to the following evaluations.

1. Electrical Conductivity

The semiconductor film device prepared was subjected to evaluation ofthe electrical conductivity of the semiconductor film, using asemiconductor parameter analyzer.

First, a voltage applied between electrodes was swept between −5 V and 5V in the state in which the semiconductor film device was not irradiatedwith light, and I-V characteristics in the dark state were obtained. Thecurrent value in the state in which a bias of +5V was applied was takenas a dark current value Id.

Then, a photocurrent value was evaluated in the state in which thesemiconductor film device was irradiated with a monochromatic light (atan irradiation intensity of 10¹³ photons). The irradiation of thesemiconductor film device with the monochromatic light was performedusing an apparatus illustrated in FIG. 4. The wavelength of themonochromatic light was systematically changed between 280 nm and 700nm. The increase in current observed under irradiation with a lighthaving a wavelength of 280 nm as compared to the dark current value wastaken as a photocurrent value Ip.

The evaluation results are indicated in Table 2.

2. Film Detachment From Substrate

Film detachment of the semiconductor film was visually evaluated withrespect to the semiconductor film devices of Examples and ComparativeExamples. Table 2 indicates whether or not film detachment was observed.

TABLE 2 Presence or absence of Ligand for Photocurrent Dark current filmexchange value Ip (A) value Id (A) detachment Example 4 2-aminoethane-7.28 × 10⁻⁵ 2.01 × 10⁻³ Absent 1-thiol Example 5 2-aminoethanol 2.99 ×10⁻⁵ 1.47 × 10⁻⁴ Absent Example 6 potassium 5.29 × 10⁻⁵ 3.10 × 10⁻⁴Absent thiocyanate Comparative ethanedithiol 1.13 × 10⁻⁵ 5.31 × 10⁻⁵Present Example 3 Comparative oleic acid measurement 1.42 × 10⁻¹² AbsentExample 4 (untreated) was impossible

As indicated in Table 2, it was found that decreasing the inter-dotdistance enabled obtainment of a high photocurrent value/dark currentvalue as compared to the semiconductor film modified with ethanedithiol(Comparative Example 3). Further, significant film detachment could beobserved with the naked eye in the case of the semiconductor filmmodified with ethanedithiol, whereas film detachment was not observedand high roughness was realized in the case of the ligands employed inExamples.

On the bases of the results of the Examples and the Comparative Examplesdescribed above, FIG. 5 illustrates the relationship between thedistance between semiconductor quantum dot particles and the electricalconductance. Since the inter-dot distance in the semiconductor quantumdots does not depend on the particle diameters of the semiconductorquantum dots but depends on the ligands coordinating to semiconductorquantum dots, the inter-dot distance is based on the values obtained bymeasurements in Examples 1 to 3 and Comparative Examples 1 and 2.

The electrical conductivity obtained in the case of ligand exchangeusing potassium thiocyanate (having an average inter-dot distance of0.42 nm) is enhanced as compared to the electrical conductivity obtainedin the case of ligand exchange using 2-aminoethanol (having an averageinter-dot distance of 0.29 nm). The reason therefor is not clear.However, it is conceivable that, in the case of potassium thiocyanate,although the average inter-dot distance is relatively large, there areregions where the inter-dot distance is locally small, and an electricalconductive property slightly higher than the value expectable from thevalue of the average inter-dot distance was obtained due to the regionsacting as main conduction paths.

However, the results indicated above basically indicate that quantum dotsemiconductor films having a smaller average inter-dot distance canrealize a higher electrical conductive property, and that, in order toreliably obtain a high electrical conductive property regardless of thecoordination form of the ligand, the average inter-dot distance ispreferably less than 0.3 nm, and more preferably less than 0.2 nm as inthe coordination with 2-aminoethanethiol.

The disclosure of Japanese Patent Application No. 2012-283030, filedDec. 26, 2012, is incorporated herein by reference in its entirety. Allpublications, patent applications, and technical standards mentioned inthis specification are herein incorporated by reference to the sameextent as if each individual publication, patent application, ortechnical standard was specifically and individually indicated to beincorporated by reference.

1. A semiconductor film comprising: a cluster of semiconductor quantum dots each having a metal atom; and ligands coordinating to respective semiconductor quantum dots, the semiconductor quantum dots having an average shortest inter-dot distance of less than 0.45 nm.
 2. The semiconductor film according to claim 1, wherein the semiconductor quantum dots have an average shortest inter-dot distance of less than 0.30 nm.
 3. The semiconductor film according to claim 1, wherein the semiconductor quantum dots have an average shortest inter-dot distance of less than 0.20 nm.
 4. The semiconductor film according to claim 1, wherein the semiconductor quantum dots are at least one selected from the group consisting of PbS, PbSe, InN, InAs, InSb, and InP.
 5. The semiconductor film according to claim 1, wherein an average particle diameter of the semiconductor quantum dots is from 2 nm to 15 nm.
 6. The semiconductor film according to claim 4, wherein the semiconductor quantum dots are PbS.
 7. A solar cell comprising the semiconductor film according to claim
 1. 8. A light-emitting diode comprising the semiconductor film according to claim
 1. 9. A thin film transistor comprising the semiconductor film according to claim
 1. 10. An electronic device comprising the semiconductor film according to claim
 1. 